Process for dispersing solids in aqueous systems



Dec. 31, 1.968 T. R. NEWMAN PROCESS FOR DISPERSING SOLIDS IN AQUEOUSSYSTEMS Filed June l5. 1967 ZorCmonmn 23040. Etoiunomu OO. .om o0 o? ONo. N M e m w w o 7 EN.: 3 J n N 7 Y M a m N .f w M |090. g l 4/ a N y/Al M lill t m Q OOOWOO- NAU-L zorzwommn moo z3: z .zoiwmu r oo. am oo.ov oN o, o n .T n N e I M .w w O00- r d W ,m s w m m w n oooa. M l/ N Hs m u Q b .m OQOOO- ...zu .Cwmv

Theodore R. Newman United States Patent O M 3,419,502 PROCESS FORDISPERSING SOLIDS IN AQUEOUS SYSTEMS Theodore R. Newman, Oak Lawn, Ill.,assignor to Nalco Chemical Company, Chicago, Ill., a corporation ofDelaware Continuation-impart of application Ser. No. 400,921, Oct. 1,1964. This application June 15, 1967, Ser. No. 646,337

S Claims. (Cl. 252-180) ABSTRACT F THE DISCLOSURE A process fordispersing and maintaining a dispersion of various solids that occur insurface waters. In the process, a particular hydrolyzedpolyacrylonitrile polymer is added to the water. It is essential thatthe hydrolyzed polyacrylonitrile polymer be of low molecular weight. Themolecular weight of the polymer should be no greater than 40,000. Theuse of low molecular weight hydrolyzed polyacrylonitrile polymersprovides much improved results over the use of other suspension ordispersion materials as well as over high molecular weight acrylonitrilepolymers of this type.

The present invention is a continuation-impart of copending and nowabandoned application Ser. No. 400,921 which was filed on Oct. 1, 1964.The invention is directed to a dispersing agent and to a method ofdispersing and suspending particles in aqueous systems.

There are a substantial number of commercial operations wherein it isimportant to maintain various solids that are found in waters in asuspended or dispersed state. If such solids cease to be suspended inthe liquid an accumulation of solids and fouling of equipment takesplace. This is true, for example, in so-called oncethrough coolingsystems. If there is a plentiful supply of water in close proximity toan industrial plant, cooling is often accomplished by passing the waterthrough heat transfer equipment and then discharging the water back toits source. Because of the large quantities of water that are employedin this system it is not feasible from an economic standpoint to filterand/or treat the water in a conventional manner. For this reason depositformation readily occurs and it is ordinarily necessary to stop the unitperiodically for cleaning purposes. This results in loss of productiontime and requires costly chemical and/ or mechanical cleaningprocedures.

In the production of paper, solids such as titanium dioxide must be keptsuspended in paper mill white waters. If such solids are deposited onthe paper, spots form which reduce the strength and quality of thepaper. Pitch that is found in white waters also should be kept insuspension. A suspending or dispersing agent which is effective in whitewaters at low concentration levels would therefore be of substantialvalue.

The shipping industry is also faced with the problem of depositformation. Ore boats that travel the Great Lakes, for example, areequipped with ballast tanks which are filled during the return trip ofthe boat in order to adjust the level of the ship in the water. Waterthat is used to iill the tanks contains silt which rapidly accumulatesin the tanks after several trips and adds significantly to the weight ofthe ships. This silt can only be removed from the tanks by dicult andtime-consuming cleaning operations.

Dispersing or suspending agents are also of importance in the steelindustry. In the production of clinkers that are used in blast furances,for example, large quantities of dust particles are formed. Theseparticles are removed in Patented Dec. 31, 1968 ICC a dust scrubber. lfthe solids are not maintained in a suspended condition in the cleaningliquid, the scrubber can only be operated for a short period of time.Under ordinary circumstances every week about thirty man hours arerequired to clean a single unit due to the fact that the solids depositon the inner surfaces of the equipment, obstructing the passage of airand reducing the vacuum or suction in the system.

Still other areas in which dispersing agents are useful includesemployment as detergent-type materials alone or with known detergents toclean out industrial reactors, in dish washers, in filter clothclean-up, etc.

The above examples illu-strate a few of the situations wherein a needexists for suspending and/or dispersing agents. These agents must beeffective at low concentration levels'. They should also be operablewith a wide variety of solid contaminants.

It is an object of the present invention to provide a dispersant havingwide applicability in liquid systems.

It is another object of the invention to provide a dispersant which iseffective at very low treatment levels.

Another object is to provide a dispersant which can be used successfullyin once-through cooling systems and which can be used to redispersesettled materials in cooling water systems.

Other objects will become apparent to those skilled in the art from thefollowing detailed description of the invention.

In general, the invention is based on the discovery that particularhydrolyzed polyacrylonitrile polymers or polymers produced by acopolymerization reaction and having substantially the same structureare unusually effective when employed as dispersing or suspension agentsin aqueous system. The hydrolyzed polyacrylonitrile polymer is usuallyapplied in the form of an aqueous solution, although other polarsolvents could be used and although it is also possible to apply theproduct in a dry form. Where an aqueous solution is employed, thesolution contains from about 1 t0 30% of hydrolyzed acrylonitrile. Thelower limit depends upon economic considerations in that it is noteconomically feasible to ship and handle too great a quantity of water.The upper limit is determined by the viscosity of the solution. Mostoften the solution will contain from about 10 to 25% hydrolyzedacrylonitrile, and more preferably from 18 to 20% of hydrolyzedacrylonitrile. As will be pointed out more :fully below, the molecularweight of the polymer is of critical importance. In general, themolecular weight of the polymer will be up to 40,000, and preferablyfrom 800 to 30,000, and more preferably from 1,000 to 20,0001. Molecularweight ranges of from 5,000 to 40,000, 10,000 to 30,000, and 15,000 to20,000 have also been used with success. The polymer when added tocooling waters and the like at dosage levels in the range of 0.1 toppm., and preferably 0.5 to 10 ppm., is capable of dispersing a widevariety of materials such as clays, quartz, iron oxides, titaniumdioxide, etc., that are found in natural surface waters, in paper millwhite waters, in cement slurries, in ceramic slips, etc.

The major constituents of suspended matter occurring in natural surfacewaters are quartz, calcit-e, kaolinite, organic matter, oxides of iron,and generally minor amounts of illite and montmorillonite. The respons-eto the subject polymeric dispersants in the molecular weight ranges setforth above has been found to be independent of the nature of thematerial in suspension. This indicates the nonspecificity of suchdispersants. It also suggests that the principal mode of attachment ofthe polymer to the particles is through hydrogen bonding. This type ofbonding occ-urs with molecules having hydrogen attached to a highlyelectronegative atom such as oxygen, nitrogen, or sulfur. The lhydrogenassociated with these hi-ghly electronegative atoms has lost much of itselectronic atmosphere and, therefore, readily shares electrons withsurface atoms of the solid particles. The formation of hydrogen bonds isa reversible phenomenon, and such systems are in a dynamic state. Bondsare constantly being formed and broken. In order to have lirm attachmentof a polymer to a surface, two requirements must be met, (1) the polymermust be large enough to provide many hydrogen bonds, and (2) theparticle must provide suflicient sites for attachment so that a givenpolymer molecule will be associated with a given particle. If thepolymer is too large, attachment can occur to more than one particle.When this happens, bridging between particles takes place and occulationoccurs. Flocculation begins to take place when the average molecularweight of partially hydrolyzed polyacrylonitrile exceeds 100,000.Dispersion does not occur with the monomeric and very low molecularweight species (less than about 250) because the period of attachment istoo small. Therefore, it can be seen that a polymer with the properfunctionality and stereoarrangement will act as a dispersant providingthe molecular weight is in the necessary range, neither too large forbridging nor too small for poor bonding.

When a polymer which is a polyelectrolyte is rmly attached to a particleit affects the gross charge (zeta potential or mobility) on theparticle. Most naturally occurring particles are negatively charged.Adsorption of a low molecular Weight anionic polymer increases thenegative charge. This increases the inter-particle repulsive forcesthereby decreasing the tendency of particles to agglomerate and settleout. This phenomenon can be studied by the use of standardmicroelectrophoresis apparatus. The addition of 1 p.p.m. of hydrolyzedpolyacrylonitrile to a suspension of 100 p.p.m. of kaolin in Chicago tapwater, increased the negative particle mobility from 1.04 to which haddropped to C., was allowed to rise at 1.5-2.0 degrees/min. up to 64 C.with partial cooling. The reaction mixture was then cooled andmaintained at C. for 31/2 hours. The solution was then heated to C. forone-half hour under slight vacuum to remove traces of unreactedacrylonitrile.

There are several methods that can be used to hydrolyze thepolyacrylonitrile. Probably the best method is to hydrolyze thepolyacrylonitrile slurry with sodium hydroxide in an aqueous medium. Oneof the problems involved in this procedure is due to the high viscositylevel that is reached during the initial stage of the hydrolysis. Inorder to avoid this difficulty, the polyacrylonitrile can be added tothe hydrolyzing medium over a 50-90 minute period. In an illustrativehydrolysis treatment, 24.4 parts by weight of a 50% NaOH solution wasplaced in a clean reaction vessel equipped with a stirrer, thermometer,and reiluX condenser and heated to C. One hundred (100) parts by weightof a 14.7% polyacrylonitrile slurry in Water was then fed continuouslyto the reaction vessel over a 50 minute period. After the last portionof polyacrylonitrile was added, the reaction was heated at 90-100 C. Atregular intervals, samples were withdrawn, cooled rapidly to roomtemperature, and submitted for evaluation. Optimum properties wereobtained with material that was hydrolyzed for 2.5-5.0 hours. Length ofhydrolysis is dependent on temperature. Analysis by infrared indicatedthat the best products were composed of 20-30% amide and 70-80% carboxylgroups. The same is true where the dispersant is prepared by acopolymerization reaction such as by the copolymerization of acrylicacid and acrylamide.

The following table (Table 1) shows the reaction conditions used inpreparing polyacrylonitriles of various molecular weights.

TABLE 1.-SYNTHESIS OF POLYACRYLONITRILES OF VARIOUS MOLECULAR W'EIGIITSIngredients (by weight) Reaction conditions Intrinsic Molecular ProductNo. v viscosity weight CN H2O (N11 .,)QSZOB NagSzOs Temp., C. Time, hrs.

1.50 p/sec./v./ cm. This corresponds to an increase in suspensionstability of from a few hours for the untreated system to over threedays. In practice, this effect will maintain the potential foulantparticles in a suspended state during their passage through a coolingsystem.

The following examples are illustrative of the present invention.

EXAMPLE 1 This example shows a suitable process for preparingpolyacrylonitrile. The ingredients that were used in the process includethe following:

Parts Ingredient- (by weight) Acrylonitrile 14.7 Water (deionized) 51.010% ammonium persulfate solution 6.9 10% sodium bisulte solution 27.4

Total 100.0

The acrylonitrile and water were mixed together in a ilask equipped withan agitator, thermometer, and condenser and warmed to 50 C. The ammoniumpersulfate solution was added to the mixture and two minutes later thesodium bisullite solution was added. Initiation of polymerizationoccurred almost immediately. The temperature,

TABLE 2.-EFFECT OF CAUSTIC CONCENTRATION 0N HYDROLYSIS OFPOLYACRYLONITRILE Mols NaOH Hydrolysis Composition per mol time, hrs.

aerylonitrile CN CONHz COZH In producing the product, molecular Weightsare determined by measuring intrinsic viscosity. The following procedurecan be used for this purpose:

Polyacrylonitrile is dissolved in dimethyl formamide at roomtemperature. Ten (10) mls. of solution is added by pipette to aCannon-Ubbelohde viscometer and the time required for a standard volumeto pass through the capillary is measured. Concentrations of solutionare chosen so that the initial time is greater than 500 sec. Theconcentration of polyaerylonitrile in DMF is decreased to .67, .50, .40,and .25 times the initial concentration. The time 5 required for thesesolutions to `pass through the capillary is measured. Specific viscosityis calculated according to the following formula:

where nsp=specic viscosity tp=time for polymer solution to pass throughcapillary ts=time for solvent to pass through capillary.

Intrinsic viscosity was obtained by graphing Cys. O'

for polyacrylonitrile,

k: 2.43 X r4 and Determining product composition Product composition canbe determined by infrared analysis by comparison of carboxylate vs.amide lines or by titration with strong acid. Of the two procedures,infrared analysis is more rapid and was used in this Work.

Standards were obtained by polymerizing recrystallized acrylamide anddistilled acrylic acid in aqueous solution as homopolymers. Thepolyacrylic acid was neutralized with CO2 free sodium hydroxide and thenmixtures of known composition were prepared from the sodium polyacrylateand the acrylamide. The mixture was then cast on an Irtran-2 plate andrun on the 11R-9. The relative heights of the 1575 and 1675 cru.1 peakswere compared at acrylic acid:acrylamide ratios of 90:10, 80:20, 70:30,60:40, 150:50, 40:60, 30:70, 20:80, 10:90 and used to estimatecomposition of the hydrolyzed polyacrylonitriles.

EXAMPLE 2 In the experiments shown in this illustrative exampledispersing agents of the subject invention were tested to determinetheir elfectiveness. The test for establishing the dispersing elect ofthe additives on fine particles in suspension involved the rotation of ascreen in a suspension. The weight of particles adhering to the screenand the relative stability of the suspension was determined at theconclusion of each test.

Equipment and materials The screen employed in this test is made of 302stainless steel. It has 40 mesh, with openings of 0.015 in. and wire of`0.01 in diameter. A section of screen 1 x 2 inches is used. A hole ispunched la in from the center of the long edge, to accommodate the @g2screw which holds the screen on the shaft. The shaft is a rod 2 incheslong, 1A inch in diameter. A %2 hole is drilled and tapped on a center1A; inch from the unattached end of the rod. Both hard rubber andstainless steel rods have been used. The rod is held in the chuck of alaboratory cone drive motor. The rod-screen assembly is rotated at 220to 230 rpm.

A 600 ml. beaker is used to hold 500 ml. of Chicago tap water containingthe suspended solids. The suspended solids employed in this work wereeither KA4- or y iron oxide. The kaolin particle size was 0.1M to p withan average of about 0.5/1.. The surface area of the kaolin wasdetermined to be 30 sq. m./gram. The iron oxide was a pigment gradefy-Fe203. The particle size averages about 0.4/1..

6 Method The screens are prepared for the test by first brushingthoroughly with a liber bristle brush in running hot water, and thenrinsing in D.I. Water. It is important to avoid the use of anydetergent. Excess water is removed by snapping the screens, and thendrying for at least one-half hour at 140 F. After drying the screens areweighed to the nearest 0.1 mg. and then mounted on the shaft of the conedrive motors.

A weighed portion of Lfoulant (250 mg.) is placed in a clean Osterizer.Five hundred (500) ml. of Chicago tap Water at room temperature is addedto the Osterizer and mixed for one minute. While the suspension ismixing, treatment is added to a 600 ml. beaker from a stock solutioncontaining 0.50 gram of active material per ml. After the suspension hasbeen mixed,4 it is transferred to the beaker and then put in placebeneath the stirring motor with the screen mounted on the shaft. Thescreen is lowered into the suspension, and the motor started.

Several tests may be run at the same time. The screen is rotated for onehour. At the end of this period, the screen is raised, and an absorbenttowel touched to the bottom edge. The screen is removed from the shaftand suspended in an oven `at F. for atleast one-half hour. The screen isreweighed to determine deposit weight. Screens are frequently examinedby a 60X microscope to verify weight gains and make visual comparison.The `deposit; Weights obtained in tests without additives were 3.4 to3.6 mg. with fy-Fe203, and 1.6 to 1.8 mg. with kaolin.

At the conclusion of the tests the beakers are set aside and observedfor stability of suspension or flocculation. A kaolin blank willcompletely settle out overnight, whereas with 1 p.p.m. of an effectiveldispersant it will remain turbid for several days. The settling rate ofiron oxide is much faster, the untreated blank drops out in less than anhour, while a treated suspension takes 2 to 4 hours.

In the table below the dispersing properties of 13 compositions are setforth. The 13 compositions tested were as follows:

Composition 1.-Acrylic acid monomer.

Composition 2.-Acrylonitrile monomer.

Composition 3.-Hydrolyzed polyacrylonitrile, molecular weight 13,000,25% amide, 75% acid.

Composition 4.--Hydrolyzed polyacrylonitrile, 19,000 molecular Weight,30% amide, 701% acid.

Composition 5.-Hydrolyzed polyacrylonitrile, 19,000 molecular weight,23% amide, 77% acid.

Composition 6.-Hydrolyzed polyacrylonitrile, 19,000 molecular weight,17% amide, 83% acid.

Composition 7.--Hydrolyzed polyacrylonitrile, 131,000 molecular Weight,30% amide, 70% acid.

Composition 8.-Hydrolyzed polyacrylonitrile, 553,000 molecular weight,30% amide, 70% acid.

Composition 9.-Hydrolyzed polyacrylonitrile, 5,000,- 000 molecularweight, 20% amide, 80% acid.

Composition 10.--Causticized lignin.

Composition 11.-Causticized mangrove tannin.

Composition 12.--Sulfonated naphthalene.

Composition 13.-Carboxy methyl cellulose.

TABLE lis-PERCENT REDUCTION IN DEPOSIT WEIGHT Comllosition Kaolin Ironoxide,

2 p.p.m. 1 p.p.m. 1 p.p.m.

0 0 0 0 0 0 75 70 83 97 70 94 97 97 94 0 55 89 55 25 66 -l- -I- -l- -I--l- 88 55 0 75 70 0 60 35 0 75 70 0 Increase in deposit Weight over thecontrol.

only such limitations should be imposed as are indicated in the appendedclaims.

I claim:

1 A process for maintaining a suspension of solids in water whichcomprises: adding to the water from about TABLE 4 Percent MolecularKaolin Iron oxide Number concentration weight 2 p.p.m. 10 p.p.m. 1p.p.ni 10 p.p.m.

1 Tenths of mg. of deposit. 2 Percent reduction in deposit weight.

The above results are set forth in FIGS. 1 and 2 of the attached drawingin which:

FIG. 1 is a graph showing the activity of l p.p.m. hydrolyzedpolyacrylonitrile wherein molecular weight and reduction in iron oxidedeposits are compared; and

FIG. 2 is a graph showing the activity of 2 p.p.m. hydrolyzedpolyacrylonitrile wherein molecular weight and reduction in kaolindeposits are compared.

The test method employed in the study of the dispersing eEect ofhydrolyzed polyacrylonitrile also evaluates the 5 effect on adherence ofsolid particles to surfaces. This essentially simulates the phenomenonoccurring in aqueous systems such as once-through cooling water systems.The charge on the suspended particles is increased. This decreases thetendency of particles to coalesce and settle out, or adhere to surfaces.

As is apparent from the above results, the molecular weight of thehydrolyzed polyacrylonitrile is of critical importance as is thecomposition of the polymer. 1f the dispersant is of the characterdescribed herein, it will act as a dispersant or suspending agent andnot as a occulating or coagulating agent. This is an importantdistinction between the subject dispersants and the flocculating agentsof the prior art.

Typical properties of a dispersant of the subject invention are given inthe following table:

Color Straw.

Odor Ammoniacal.

Viscosity 70-100 c.p.s.

Density 1.166 g./cc. (26 C.)

Pounds per gallon 9.75.

Corrosivity 3.2/0.25 m.p.y.

Drum liners Rheem 96720 IC136.

Flash point High (water is the only solvent used).

Freezing point Not determined, but no change at F.

The subject dispersants maintain the particle size of the suspendedsolids at a substantially constant level. The process is operable onparticles falling within a wide size range. The diameter of theparticles will be between about 0.1 and 2000 microns, more often will bebetween 0.2 and 1000 microns, and still more often will be between and800 microns.

Obviously many modifications and variations of the invention `ashereinbefore set forth may be made without departing from the spirit andscope thereof, and therefore 0.1 to 100 p.p.m. of a hydrolyzedpolyacrylonitrile having a molecular weight of 800 to 40,000 and havingside groups 2030% of which are amide groups and from 7-80% are carboxylgroups, said polymer being further characterized by its substantialfreedom from flocculating properties.

2. A process as in claim 1 wherein said polymer` has a molecular weightof from 800 to 30,000.

3. A process as in claim 1 wherein said polymer has a molecular weightof from 5,000 to 30,000.

4. A process for maintaining an aqueous suspension of. solids in waterwhich comprises: adding to said water from about 0.5 to 10 p.p.m. of ahydrolyzed polyacrylonitrile, said polymer having a molecular weight offrom 5,000 to 40,000 and having from 20-30% amide groups and from -80%carboxyl groups, said polymer being further characterized by itssubstantial freedom from flocculating properties, said solids beingcharacterized by their ability to become attached to said polymerthrough hydrogen bonding.

5. A process for preventing fouling and corrosion due to deposits inwater cooled industrial heat exchangers and cooling systems whichcomprises: treating the cooling water passing through said coolingsystem by adding to said water from labout 0.1 to p.p.m. of a hydrolyzedpolyacryonitrile, said polymer having a molecular weight of 800 to40,000 and having from 20-30% amide groups and from 70-80% carboxylgroups, said polymer being further characterized by its substantialfreedom from occulating properties.

6. A process as in claim 5 wherein said polymer has a molecular weightof from 5,000 to 30,000.

7. A process for dispersing solids in an aqueous system which comprises:adding to said aqueous system containing accumulated solids from about0.1 to 100 p.p.m. of a hydrolyzed polyacrylonitrile having a molecularweight of 800 to 40,000 and having side groups 20-30% of which are amidegroups and from 70-80% are carboxyl groups.

8. A process as in claim 7 wherein the diameter of the particles to bedispersed varies between about 0.1 and 2,000 microns.

References Cited UNITED STATES PATENTS 3,085,916 4/1963 Zimmie et al.210-58 LEON D. ROSDOL, Primary Examiner. W. E. SCHULTZ, AssitantExaminer.

U.S. Cl. X.R. 134-22; 210-58 A UNITED STATES PATENT OFFICE CERTIFICATEOF CORRECTION Patent No. 3,419,502 December 31, 1968 Theodore R. NewmanIt is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column Z, line 34, "system" should read systems Column 8, line 29, "7"should read 70 Signed and sealed this 21st day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents

